The present invention is directed to a reactor shroud shield and method for using it, and more particularly to a gamma-ray attenuating shield which can be installed to protect technicians working in the region of a reactor head.
A nuclear reactor typically includes a reactor vessel which is disposed in a concrete well located in the floor of a reactor room. A reactor head is bolted to the reactor vessel, which contains water and nuclear fuel housed in an array of fuel assemblies. During operation the nuclear fuel heats the water, which is circulated by pipes from the reactor vessel to steam generators and then back to the reactor vessel for reheating. In order to regulate the degree of water heating and control the nuclear reaction, control rods of neutron absorbing material are movably mounted within the reactor vessel. Control rod drive mechanisms mounted on the reactor head are used to adjust the control rods, with the control rod drive mechanisms and various control and sensor devices being housed within a shroud extending upward from the reactor head. It will be understood that the term "shroud" refers to the housing for the control rod drive mechanisms, although this housing may occasionally be known by other terms, such as lifting skirt. Control rod drive mechanisms are disclosed, for example, in U.S. Pat. No. 3,766,006.
During refueling, the nuclear reaction within the vessel is quenched and technicians wearing protective clothing unbolt the reactor head by removing nuts from studs so that the head can be removed from the reactor vessel. These nuts are large and heavy, and a stud tensioner hoist mounted on a circumferential track is available to assist the technicians in handling the nuts and tools. After the reactor head has been bolted, the reactor room is flooded with water to provide shielding and the reactor head, control rod drive mechanisms, shroud, etc., are hoisted by lifting rods attached to the head to expose the depleted fuel assemblies. The depleted fuel assemblies are removed, under water, to a pool of borated water for short-term storage. The reactor is then refueled and the head assembly, complete with control rod drive mechanisms, etc., is set in place on top the reactor vessel. After the water is drained from the reactor room technicians wearing protective clothing re-enter the reactor room to securely bolt the reactor head to the reactor vessel so that operation can continue.
The reactor head itself is a hemispherical body typically made of steel, perhaps 18 cm thick, and substantially attenuates gamma-rays from inside the reactor vessel. However the control rod drive mechanisms, during the course of time, tend to accumulate a residue of fission products. Since the control rod drive mechanisms are shielded by the shroud alone, and not by the reactor head, the technicians in the reactor room during the refueling operation are exposed to radiation from the control rod drive mechanisms. While this exposure is a modest one, the steadily increasing safety standards in the nuclear industry have made it desirable to provide additional shielding to the technicians during the refueling operation.
U.S. Pat. No. 4,432,932 provides additional shielding by suspending quilted blankets of lead wool from short mounting arms fixedly attached to the shroud. The technicians install the blankets before undertaking the longer task of bolting or unbolting the reactor head. However, such blankets are unwieldy, and in practice may weigh substantially more than the technicians who install them. Even if the stud tensioner hoist is used to manipulate the blankets as they are being hung or removed, the blankets must still be muscled laterally by the distance between the stud tensioner hoist and the mounting arms on the shroud. This not only increases fatigue, it slows the blanket installation process and consequently increases the exposure to radiation. Moreover the heavy weight and awkward movements that are necessary increase the possibility of slips or other accidents.